It is known to use free radical generating compounds to modify a product, such as a polymer composition via a radical reaction.
Free radical generating compounds are used e.g. to initiate (a) crosslinking in a polymer, i.a. primarily a formation of interpolymer crosslinks (bridges) by radical reaction, (b) grafting in a polymer, i.e. introduction of compounds to a polymer chain (to backbone and/or side chains) by radical reaction, and (c) visbreaking a polymer, i.e. modification of melt flow rate (MFR) of a polymer by radical reaction. These polymer modifications are well known in the art.
When added to a polymer composition, free radical generating compounds act by generating radicals, typically by decomposing to radicals, under conditions which enable the radical formation. The decomposed radicals initiate further radical reactions within a polymer composition. The resulting decomposition products of the free radical generating compound are typically a result of several reactions of the decomposition products of initial radical forming reaction. Said resulting decomposition products typically remain in the modified polymer and may include detrimental, undesired decomposition products. Peroxides are very common free radical generating compounds used i.a. in the polymer industry for said polymer modifications. The resulting decomposition products of peroxides may include volatile by-products. For example, dicumylperoxide, which is commonly used peroxide in polymer field, decomposes i.a. to methane, acetophenone and cumylalcohol during the radical formation step, e.g. during a crosslinking step. The formed gaseous methane (CH4) is flammable, explosive and volatile and thus a risk in a working environment.
In wire and cable applications a typical cable comprises at least one conductor surrounded by one or more layers of polymeric materials. In some power cables, including low voltage (LV), medium voltage (MV), high voltage (HV) and extra high voltage (EHV) cables, said conductor is surrounded by several layers including an inner semiconductive layer, an insulation layer and an outer semiconductive layer, in that order. The cables are commonly produced by extruding the layers on a conductor. The polymer material in one or more of said layers is then typically crosslinked to improve i.a. heat and deformation resistance, creep properties, mechanical strength, chemical resistance and abrasion resistance of the polymer in the layer(s) of the cable. The free radical generating compound, such as a peroxide, is typically incorporated to the layer material prior to the extrusion of the layer(s) on a conductor. After formation of the layered cable, the cable is then subjected to a crosslinking step to initiate the radical formation and thereby crosslinking reaction.
The decomposition products of the free radical generating compound remain mostly captured within the cable layer after crosslinking. This can cause problems in view of the cable manufacturing process as well as in view of the quality of the final cable.
Accordingly, after crosslinking the cable must be cooled with great care to prevent the gaseous volatile decomposition products like methane forming voids within the polymer layer. These voids have typically an average diameter of between 10 to 100 μm. Partial discharges can take place in such voids within a cable that is subjected to an electrical field and thereby reduce the electrical strength of the cable.
Particularly the MV, HV and EHV power cables must have high layer quality in terms of safety during installation and in end use thereof. In service, volatile decomposition products in a cable resulting from a crosslinking step can create a gas pressure and thus cause defects in the shielding and in the joints.
For the above reasons the volatile decomposition products, such as methane e.g. where dicumylperoxide is used, are conventionally reduced to a minimum or removed after crosslinking and cooling step. Such removal step is generally known as a degassing step. The degassing step is time and energy consuming and is thus a costly operation in a cable manufacturing process. Degassing requires large heated chambers which must be well ventilated to avoid the build-up of, for example, flammable methane and ethane. The cable, typically wound to cable drums, is normally degassed at elevated temperature in the range of 50-80° C., e.g. 60-70° C., for lengthy time periods. At these temperatures however, thermal expansion and softening of the insulation can occur and lead to undue deformation of the formed cable layers resulting directly in failures of the cable. The degassing of MV, HV and EHV cables with high cable weight needs thus often be carried out at decreased temperatures.
U.S. Pat. No. 5,618,900 describes a polymerisation process of monomers using an initiator which can be a ene-diyne compound amongst others. The amount of the initiator used in the polymerisation is stated to be 10 to 2000 ppm.
Accordingly, there is a need to find new solutions to overcome the prior art problems.